patch 8.2.1024: Vim9: no error for using "let g:var = val"

Problem:    Vim9: no error for using "let g:var = val".
Solution:   Add an error.
diff --git a/src/scriptfile.c b/src/scriptfile.c
index abfa3b5..caeab19 100644
--- a/src/scriptfile.c
+++ b/src/scriptfile.c
@@ -1873,7 +1873,7 @@
     nr = getdigits(&eap->arg);
     if (nr == 0 || *eap->arg != NUL)
 	emsg(_(e_invarg));
-    else if (nr > 4)
+    else if (nr > SCRIPT_VERSION_MAX)
 	semsg(_("E999: scriptversion not supported: %d"), nr);
     else
     {